The proposed investigations will examine the structural changes which occur to the capillaries in the rat dentate gyrus: 1) during development, maturation, and aging and 2) following adjacent neuronal denervation induced at three different age periods -- early juvenile, early and late adulthood. This study is designed to document changes in the morphological substrates responsible for the blood-brain barrier. Thin-section and freeze fracture electron microscopic techniques will be employed to analyze the endothelial cell, pericyte, and astroglial "end-feet" organelle content and membrane morphology. The degree of pinocytotic activity will be determined, morphological changes in the endothelial intercellular junctions (tight junctions) described, and the endothelial tubulo-channels studied. Significant differences in the rate of synaptic recovery in animals lesioned at various ages have been reported. This study intends to test the hypothesis that capillary permeability changes progressively during the developmental period, remains relatively constant during adulthood, and changes again during late adulthood (ages which show substantially different capacities for synaptic recovery), and that the degree of permeability present at these ages influence the capacity of neurons to grow new synaptic connections following partial denervation. The long-term objective of this proposal is to increase our understanding of the role capillaries play in both aging and neural recovery following brain damage.